Coated ferrous substrate

ABSTRACT

A steel sheet has a composite coating thereon comprising a layer of chromium 5 to 100 microinches thick contiguous to the steel and a layer of aluminum 100 to 1,500 microinches thick contiguous to the layer of chromium. The chromium is deposited on the sheet while the steel is at a temperature within the range of 1,200* to 1,700* F.

United States Patent Seiler 1 Apr. 4, 1972 [54] COATED FERROUS SUBSTRATE2,957,782 10/1960 Boiler ..29/196.6 x

3,436,805 4/1969 Friske et al.. [72] Inventor Bernard Bethlehem2,856,333 10/1958 Topelian ..29/197 [73] Assignee: Bethlehem SteelCorporation [22] Filed: Oct. 14 1969 Primary Examiner-L. DewayneRutledge ASSISIG"! Examiner-G. K. White [21] Appl. No.: 866,290Attorney-Joseph J. OKeefe 52 us. on ..29/196.6, 29/1962, 29/197 ABSTRACT[51] .....B32b 15/18, B32b 15/20 A Steel sheet has a com posite coatingthereon comprising a [58] Field of Search ..29/ 196.2, 196.6, 197 layerof chromium 5 to 100 microinches thick contiguous to 56 R f C" d thesteel and a layer of aluminum 100 to 1,500 microinches 1 e erences ethick contiguous to the layer of chromium. The chromium is UNITED STATESPATENTS deposited on the sheet while the steel is at a temperaturewithin the range of 1,200 to 1,700 F. 2,917,818 12/1959 Thomson..29/l96.2 3,323,881 6/1967 Nelson et al. ..29/l96.6 X 4 Claims, 2Drawing Figures Cf THICKNESS (MICROINCHES) Patented April 4, 1972 2Sheets-Sheet l mmm OON

OOO. 000m 000m OOO 000m INVENTOR Bernard C. Sei/er Patented April 4,1972 2 Sheets-Sheet 2 Fig. 2

0 O u M M o M W 2 .7 O E O N P Q o O 7 P 0%0 0 0 H w .1. n m U q o a O m0 o O 8 O 0 6 m H 0 a U 0 W H 4 m M m O u M E M o O H A u 2 W 0 O O O o0 O O O O 9 8 7 6 5 4 3 2 I ZQWOEIOU .rZwu mum TEMPERATURE (F) INVENTORBernard C. Sei/er COATED FERROUS SUBSTRATE BACKGROUND OF THE INVENTIONThis invention relates to a ferrous substrate having a coating thereon,and more particularly to a steel sheet having a composite coating ofchromium and aluminum thereon.

Steel articles which are exposed to the atmosphere, e.g., sheds,buckets, garbage cans, etc., are generally coated with another metal inorder to extend the service life of the article. In addition toprotecting the steel from corrosion, it is essential for the coating tobe ductile and to adhere satisfactorily to the steel base so that thearticle can be readily formed from the coated steel base.

In the past, steel sheets have been coated with metals such as aluminumto provide a coated sheet having resistance to atmospheric corrosion.While articles made from such coated sheets have a fairly long servicelife, it is desirable to provide a coated steel sheet having an evenlonger service life.

It is an object of this invention to provide a coated ferrous substratehaving a service life longer than that of steel sheets coated solelywith aluminum, said coated ferrous substrate having excellent ductility.

It is a further object to provide a method for producing such a coatedferrous substrate.

SUMMARY OF THE INVENTION 1 have discovered that the foregoing objectscan be attained by heating a ferrous substrate to a temperature withinthe range of 1,200" to l,700 F. and depositing a layer of chromium onsaid substrate while said substrate is within said temperature range,said layer having a thickness of from 5 to 100 microinches. A layer ofaluminum is then deposited on said layer of chromium.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing the life ofcoated steel sheets in the ASTM B-l l7-62 salt fog test.

FIG. 2 is a graph showing the ductility of the coated steel sheet as afunction of the temperature of the steel during the deposition of thechromium thereon.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows the results of theASTM B-l 17-62 salt fog test, said test being a general indication ofthe samples resistance to atmospheric corrosion. The abscissa is interms of thickness of the chromium layer, in microinches, while theordinate is in terms of hours exposure per mil of total coatingthickness until failure. The aluminum varied in thickness from about 100to 700 microinches. In all cases, both the chromium and the aluminumwere deposited on sheet steel samples by vacuum vapor deposition. Thechromium was deposited while the steel was above l,000 F while thealuminum was deposited while the steel was at ambient temperature. Inorder to avoid any confusion which might result from plotting the entiremass of data, the results for a particular composition range wereaveraged and plotted as a straight line extending over the compositionrange from which the data were obtained. Above each line is the numberof samples averaged to obtain said line.

As is shown in FIG. 1, if the coating comprisesonly aluminum, i.e., ifthere is no layer of chromium intermediate the steel and the aluminum,the life in the salt fog test averaged about 1,500 hours per mil ofcoating thickness. However, a layer of chromium only 5 microinches thickbetween the steel and the aluminum resulted in a salt fog test life ofabout 4,000 hours per mil of total coating thickness, said life beingmore than double that for aluminum alone. The maximum life occurred atabout 20 microinches of chromium, and was about 8,000 hours per mil oftotal coating thickness. Surprisingly, as the layer of chromiumincreased in thickness beyond 20 microinches, the life in the salt fogdecreased. As can be seen, the life in the salt fog had fallen to about1,000 hours per mil of total coating thickness at 200 microinches ofchromium.

Between 5 and microinches of chromium, the salt fog life is consistentlyabove 3,500 hours per mil of total coating thickness, and this range ofchromium thickness, marked A" in FIG. 1, constitutes the preferred rangeof the invention.

While the coatings may be applied by any known methods, e.g.,electrodeposition or hot dip, they are preferably applied by vacuumvapor deposition. In order to get satisfactory ductility, it was foundnecessary to deposit the chromium on the steel sheet while the steel wasat a temperature of at least l,200 F. In Table I, below, the ductilityof samples coated with chromium was tested by stripping the steel basefrom the coating and bending the coating over on itself. If no cracksoccurred, the coating was considered ductile.

TABLE I Temperature of Steel FIG. 2 shows the ductility of steel sheets,coated with a layer of aluminum over a layer of chromium, as a functionof the temperature of the steel sheet during the deposition of thechromium thereon. The abscissa is in terms of the temperature of thesteel, immediately prior to the deposition of chromium thereon, whilethe ordinate is in terms of percentage corrosion in the modified blottertest. In this test, beer can ends (2.687 inch in diameter) are punchedby conventional methods from the coated sheet to be tested. These endsare closed over can bodies with the coating on the outside. The cans arethen placed on a wet blotter with the test coating thereagainst. Theblotter is kept moist by maintaining both ends thereof in a reservoir ofdistilled water. A weight is placed on top of the cans to ensureintimate contact between the cam rim and the blotter. After 48 hours,the cans are removed and the length of the corrosion products fromexposed steel along the rim of the can ends is measured and expressed asa percentage of the circumference of the can end.

The rim of the can end making contact with the blotter is deep drawnduring the end punching operation and is subjected to a mild ironing anda 3T bend during the closing operation on the can body. Steel will bereadily exposed if the coating is very brittle.

As is shown in FIG. 2, the coating is quite brittle at a chromiumdeposition temperature of 200 F., but as the temperature of the steelincreases the degree of brittleness rapidly decreases until a low levelis reached at about 1,200" P. This low level of brittleness ismaintained at temperatures up to at 1,500" F. Higher temperatures werenot investigated, inasmuch as strip handling becomes quite difficult attemperatures above l,500 F. However, it appears from an extrapolation ofthe curve that the ductility will remain satisfactory up to at leastl,700 F.

In view of the foregoing, the ferrous substrate is heated to atemperature within the range of 1,200 to 1,700 F., and the chromium isdeposited thereon, preferably by vacuum vapor deposition, while thesubstrate is within said range. The aluminum coating is then applied,also preferably by vacuum vapor deposition. Preferably, the aluminum is100 to 1,500 microinches thick.

A preferred ferrous substrate for the subject coatings comprises a sheetsteel consisting essentially of 0.03 to 0.15 weight percent carbon, 0.20to 0.60 weight percent manganese, balance iron. By balance iron, I donot wish to exclude normal impurities such as 0.003 to 0.020 weightpercent phosphorus and 0.010 to 0.050 weight percent sulfur. If

desired, the carbon in the steel may be stabilized by means of titanium.

EXAMPLE I As a first specific example of my invention, atitanium-stabilized steel strip 0.014 inch thick was vapor degreased intrichlorethylene. The steel consisted essentially of 0.038 weightpercent carbon, 0.34 weight percent manganese, 0.43 weight percenttitanium, balance iron. The strip was then cleaned cathodically in a hotPennsalt solution. The cleaned strip was preheated to l,500 F. in avacuum coating chamber evacuated to a pressure of about torr., and 8microinches of chromium were deposited thereon by conventional vacuumvapor deposition techniques.

The coated strip was next re-wound in its original position and 164microinches of aluminum were deposited, at ambient temperature, on topof the layer of chromium. Two test panels were cut from the strip. Oneof said panels was deformed by the Ericksen Cup Tester to the maximumcup depth without rupturing the steel. Both panels were then subjectedto the above-referred-to salt fog test. Inasmuch as the strip was coatedon only one side thereof, the backs and the edges of the panels wereprotected with electroplaters tape during said test.

The cupped panel had a life of 1,080 hours, or 6,280 hours per mi] oftotal coating. The other panel had a life of 890 hours, or 5,170 hoursper mil of total coatingv EXAMPLE ll As a second specific example of myinvention, a low carbon steel strip 0.0105 inch thick and consistingessentially of 0.07 weight percent carbon, 0.3 weight percent manganese,balance iron, was coated with 15 microinches of chromium at 1,200 F. bythe same procedures outlined in Example I. A layer of aluminum 172microinches thick was then vacuum vapor deposited on the layer ofchromium. As in Example 1, two test panels were cut from the strip, onepanel was cupped, and both were then tested in salt fog. The cuppedpanel had a life of 980 hours, or 5,700 hours per mil of coating. Theother panel had a life of 1,240 hours, or 7,220 hours per mil of totalcoating.

1 claim:

1. An article comprising a ferrous substrate having a coating thereon,said coating comprising a layer of chromium 5 to microinches thickcontiguous to said substrate and a layer of aluminum contiguous to saidchromium, said article having a minimum life in the ASTM B-l 17-62 saltfog test of 3,500 hours per mil of total coating thickness.

2. An article as recited in claim 1, in which said aluminum is 100 to1,500 microinches thick.

3. An article as recited in claim 1, in which said ferrous substrate issheet steel.

4. An article as recited in claim 3, in which said steel consistsessentially of 0.03 to 0.15 weight percent carbon, 0.20 to 0.60 weightpercent manganese, balance iron.

2. An article as recited in claim 1, in which said aluminum is 100 to1,500 microinches thick.
 3. An article as recited in claim 1, in whichsaid ferrous substrate is sheet steel.
 4. An article as recited in claim3, in which said steel consists essentially of 0.03 to 0.15 weightpercent carbon, 0.20 to 0.60 weight percent manganese, balance iron.